(Blender 2.8) Why is my smoke simulation animation not visible in flow type?
For the smoke simulation, you need two objects.

One is the source of the smoke (Flow), and the other is the volume in which the smoke will appear (Domain).

The Flow object MUST be inside the Domain object, otherwise you will never see anything.

If you use the Quick Smoke option, it makes the selected object the Flow object, and automatically adds the Domain for you (typically a cube).  It's worth experimenting with Quick Smoke in a separate blend file or collection, just to see how it works.